Hey Micheal, thanks for signing up! As you noticed, setting a password is optional and I think you can log in with any of the services and as long as the email address matches it will bind them to the same forum account.
I think they could improve this process a little since indeed when using an invite URL you are already logged in and at that point there is no option to use another service instead of setting a password.
Welcome eka, glad to hear you like the new forum!
I am of course a little afraid to annoy some people with invites, but on the other hand it seemed like the best way to make people aware of this new place and try it out a bit.